1. Field of the Invention
The present invention relates generally to a double side printing apparatus for printing on double sides of a recording medium and, more particularly, to a double side printing apparatus for minimizing a printing positional deviation between right and reverse sides thereof.
2. Related Background Art
A printing apparatus has been widely utilized as an output apparatus of a computer. An electrophotographic apparatus capable of printing on an ordinary sheet of paper has been utilized as the printing apparatus. In response to a demand for saving natural resources in recent years, a double side printing (perfecting) apparatus for printing on double sides of the sheet has been required. Then, an apparatus provided with both a mechanism for printing on the right side of the recording medium and a mechanism for a printing mechanism for printing on the reverse side of the recording medium, is required for increasing a printing speed.
FIG. 9 is an explanatory view showing the prior art.
The perfecting apparatus includes a printing unit 9b for printing on the right side of a sheet of recording paper PP(which is hereafter simply referred to as the sheet PP), and a printing unit 9a for printing on the reverse side of the sheet PP. The reverse side printing mechanism 9a has a photosensitive drum 90. The photosensitive drum 90 is charged by a pre-charger 91 and thereafter exposed to a light image by an exposing unit 92. The exposing unit 92 serves to scan one line on the photosensitive drum 90. An electrostatic latent image corresponding to the light image is thereby formed on the photosensitive drum 90. The latent image on the photosensitive drum 90 is developed by a developing unit 93. Then, the developed image on the photosensitive drum 90 is transferred onto the sheet PP by a transferring unit 94. Thus, the image is printed on the reverse side of the sheet PP.
The printing on the right side of the sheet PP is likewise performed. To be specific, the right side printing mechanism 9b has a photosensitive drum 95. The photosensitive drum 95 is charged by a pre-charger 96 and thereafter exposed to a light image by an exposing unit 97. An electrostatic latent image corresponding to the light image is thereby formed on the photosensitive drum 95. The latent image on the photosensitive drum 95 is developed by a developing unit 98. Subsequently, the developed image on the photosensitive drum 95 is transferred onto the sheet PP by a transferring unit 99. Thus, the image is printed on the right side of the sheet PP.
The perfecting apparatus has the two printing mechanisms which each prints on the single side of the sheet PP. It is required that a printing control unit for controlling the printing mechanism be provided for the printing mechanism. The printing control unit performs the control so that the exposing unit of the printing mechanism executes a one-line exposure in accordance with a printing start timing.
The printing apparatus has one printing control unit and therefore is, it has hitherto been practiced, provided with the printing control units for controlling the two printing mechanisms.
There arise, however, the following problems inherent in the prior art.
First, a method of providing a common control unit for controlling the two print mechanisms involves desinging a common print control unit different from a print control unit of a one-side printing apparatus for printing on one side. Therefore, the print control unit of the perfecting apparatus can not be used for the print control unit of the one-side printing apparatus, resulting in such a problem that the costs light increase.
Second, according to a method of providing each individual printing mechanism with the print control unit, the above print control unit can be used for the print control unit of the one-side printing apparatus, which is advantageous in terms of the costs. However, the two print control units are independently controlled, and therefore the print start timing for the right side is asynchronous with the print start timing for the reverse side, which conduces to such a problem that a print position on the right side deviates from a print position on the reverse side.